The invention relates to extreme pressure lubricating oils, particularly alkali-metal borate-containing lubricants. More particularly, the invention relates to the finding that alkali-metal borate lubricants are greatly improved by addition to the borate lubricants of a combination of phosphates (sulfur-free), monothiophosphates and dithiophosphates in a critical ratio.
Alkali-metal borate-containing lubricants are well known in the art for their usefulness as extreme pressure lubricating oils. See, for example, U.S. Pat. Nos. 3,313,727, 3,565,802, 3,819,521, 3,846,313, 3,853,772, 3,907,691, 3,912,639, 3,912,643, 3,912,644, 3,997,454, and 4,089,790.
These patents also teach that the antiwear and load-carrying properties of the lubricants can be improved through the use of phosphate additives, particularly the zinc dithiophosphates and amine salts of dithiophosphoric acid (U.S. Pat. Nos. 3,997,454 and 4,089,790).
It is one object of the present invention to provide an alkali-metal borate-containing lubricant having improved wear and load-carrying properties. The improved properties are obtained through the use of a critical ratio of phosphate, monothiophosphate and dithiophosphate additives.